<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <title>Title</title>
</head>
<body>
    <form action="add.do" method="post">
        <input type="hidden" name="flag" value="add">
        <input type="text" name="username" >
        <input type="text" name="pwd" >
        <select name="sex">
            <option value="boy">boy</option>
            <option value="girl">girl</option>
        </select>

    <input type="submit" value="Submit">
</form>
</body>
</html>